here's a valid f-string:

>>> f'{'}'}' '}'

Huh? I remember learning the rule that you can't nest quotes inside fstrings if they are the same kind (unlike in bash) - e.g

  f"{mydict["foo"]}"

would be a syntax error, but

  f"{mydict['foo']}"

or

  f'{mydict["foo"]}'

would be valid.

The reason being the same like for the rstring weirdness: The lexer comes first and identifies the string literal, then for fstrings, the python parser is invoked again for each {...} expression to parse it.

This is unlike other nested expressions, which are already split up by the lexer and then parsed in one go.

Did that change at some point?